In particular, port cities like New York, like Boston, like Philadelphia, were particularly susceptible to outbreaks of infectious diseases because these were places where ships coming in from other parts of the world also brought travelers who would have been exposed to these diseases in the cities where they had departed. Cholera, for example, had largely been restricted to the Far East until advances in shipping shortened the travel time between the world's major ports. And because cholera has a very short incubation period and a rapid course, the fact that there was more rapid transportation between the Far East and the U.S. increased the spread of this disease.

Now, one of the major changes that occurs toward the end of the 19th century, as people tried to figure out how to control infectious diseases, was the introduction of the germ theory of disease. It became clear that impure water, crowding, poor housing, spoiled food, and other environmental conditions were contributing factors to high rates of disease in the cities. As I said also, people believed that immoral behavior also facilitated the spread of infectious diseases. But during moments of crisis, some also sought to focus on personal hygiene, child care practices, as also leading culprits in the spread of disease. But these differing explanations for high disease rates have profound implications for prevention planning. The only means of preventing these diseases at this time was to clean up the city, improve the housing stock, reduce overcrowding, provide better sanitation, et cetera.

Now, this environmental emphasis implied the need for massive social investment and a change in social and economic relationships; that is, to provide better housing for people, to use taxpayers' funds to build major systems, like sewerage systems. One of the causes of diseases that people believed at this moment also implied that there had to be also an emphasis on personal hygiene, and placing the responsibility for hygiene on individuals. This led to a kind of blaming the poor and the sick for their diseases as opposed to saying that diseases lay outside of individuals.

But slowly, as medical experts carefully analyzed transmission of diseases, of these infectious diseases, the belief grew that there were specific causes, specific pathogens associated with specific diseases, and these pathogens turned out to be microscopic organisms or bacteria. So beginning in [the] 1870s and 1880s, researchers abroad discovered the microorganisms associated with tuberculosis, with cholera, with typhoid, and with diphtheria. Now, American physicians were initially a little bit skeptical that a little microbe, a little bacteria, could actually cause diseases that wreaked such havoc in the cities, but with the development of diphtheria antitoxin in [the] 1890s, which dramatically reduced the mortality associated with this disease, a number of physicians were finally convinced that the germ theory was correct.

Now, for public health experts, the germ theory proved to be especially rewarding, making visible the agents of disease in water, food, and blood. And by the 1910s, physicians were provided with precise tests to identify a variety of diseases. And so the germ theory seemed to promise a precision, not only in the diagnosis of disease, but also in the prevention of disease. And proponents of public health adopted a new approach to their work. They rejected the idea that they had to simply clean up the environment and get rid of the filth. The new public health emphasized that germs were spread by personal contact, and the new practices centered on educating individuals in their responsibility for the prevention of disease. In sum, the focus of public health work changed from citywide sanitation and disease control to closer observation of individuals, their habits, and their contagiousness.